Synthesis, spectroscopic and luminescence properties of Ga-doped γ-Al2O3.

Gallium-doped aluminum oxide (Al1-xGax)2O3 with γ-Al2O3 (spinel) structure has been synthesized by the precursor method using the formate Al1-xGax(OH)(HCOO)2 as a precursor. The examination of Al1-xGax(OH)(HCOO)2 (x = 0.0, 0.1, 0.2, 0.3, 0.4) was carried out by X-ray powder diffraction, Infrared, Raman spectroscopy and differential-thermal methods. The solid solutions γ-(Al1-xGax)2O3 with х≤0.2 have been synthesized by thermolysis of precursors in helium atmosphere at 700 °C; they exhibit white-blue emission under UV excitation, whose intensity lowers with increasing dopant concentration. As an independent method, the DFT calculations confirmed thermodynamically the stability field of γ-(Al1-xGax)2O3 solid solutions and the NMR data on relative abundance of Al and Ga within the tetrahedral and octahedral sites in the metal sublattice. Furthermore, the structural and thermodynamic properties of carbon-containing impurities within these compounds were suggested theoretically as possible models of luminescence emission centers. The experimentally observed Ga-dependent quenching of luminescence was explained using the competition between C2p and Ga4p states within the band gap of γ-Al2O3.

Do cement nanotubes exist?

Using atomistic simulations, this work indicates that cement nanotubes can exist. The chemically compatible nanotubes are constructed from the two main minerals in ordinary Portland cement pastes, namely calcium hydroxide and a calcium silicate hydrate called tobermorite. These results show that such nanotubes are stable and have outstanding mechanical properties, unique characteristics that make them ideally suitable for nanoscale reinforcements of cements.

Structure and stability of molybdenum sulfide fullerenes.

MoS2 nanooctahedra are believed to be the smallest stable closed-cage structures of MoS2, i.e., the genuine inorganic fullerenes. Here a combination of experiments and density functional tight binding calculations with molecular dynamics annealing are used to elucidate the structures and electronic properties of octahedral MoS2 fullerenes. Through the use of these calculations MoS2 octahedra were found to be stable beyond nMo > 100 but with the loss of 12 sulfur atoms in the six corners. In contrast to bulk and nanotubular MoS2, which are semiconductors, the Fermi level of the nanooctahedra is situated within the band, thus making them metallic-like. A model is used for extending the calculations to much larger sizes. These model calculations show that, in agreement with experiment, the multiwall nanooctahedra are stable over a limited size range of 104-105 atoms, whereupon they are converted into multiwall MoS2 nanoparticles with a quasi-spherical shape. On the experimental side, targets of MoS2 and MoSe2 were laser-ablated and analyzed mostly through transmission electron microscopy. This analysis shows that, in qualitative agreement with the theoretical analysis, multilayer nanooctahedra of MoS2 with 1000-25 000 atoms (Mo + S) are stable. Furthermore, this and previous work show that beyond approximately 105 atoms fullerene-like structures with quasi-spherical forms and 30-100 layers become stable. Laser-ablated WS2 samples yielded much less faceted and sometimes spherically symmetric nanocages.